The majority of television tuners used today are discrete single conversion tuners. FIG. 1 illustrates in partial block diagram and partial schematic form such a discrete single conversion television tuner 20 known in the prior art. Television tuner 20 has an input for receiving a radio frequency (RF) signal labeled “RF INPUT” from an antenna or cable source (not shown) having channels in the range of 48 megahertz (MHz) to 870 MHz. A tracking bandpass filter 22 receives the RF INPUT signal and attenuates undesired channel energy to provide a filtered signal to an input of a low noise amplifier labeled “LNA” 24. An RF synthesizer 28 controls a variable local oscillator (LO) 30 to provide a mixing signal in the range of 95 to 911 MHz. The mixing signal is combined with the output of LNA 24 in a mixer 26, which mixes the desired channel to an intermediate frequency (IF) of 44 MHz. The output of mixer 26 is amplified in a programmable gain amplifier (PGA) 32 and is filtered in an IF filter 34 having a center frequency at the conventional IF of 44 MHz and having a passband of 6 MHz. Thus IF filter 34 provides an output signal containing the desired channel and having frequency content primarily from 41 MHz to 47 MHz.
Discrete single conversion tuners such as tuner 20 suffer several disadvantages. Tuner 20 requires a large amount of circuit board space due to the large number of discrete components. It also requires RF expertise to lay out the circuit board to avoid undesirable signal cross coupling and interference. Tracking bandpass filter 22 needs manual calibration, increasing production cost. Also the performance of tuner 20 varies significantly over temperature.
It has long been thought that a silicon-based television tuner could be manufactured cheaper and with more stable performance than a discrete tuner and that silicon-based television tuners would ultimately replace discrete television tuners. Unfortunately, existing silicon-based television tuners do not perform as well as discrete tuners and have not become significant in the marketplace.
FIG. 2 illustrates one known existing silicon-based television tuner 40. Tuner 40 uses a so-called “up/down” or double conversion architecture. Tuner 40 includes an LNA 42, an up conversion mixer 44, an RF synthesizer 46, a local oscillator 48, a surface acoustic wave (SAW) filter 50, a PGA 52, a down conversion mixer 54, a local oscillator 56, and an IF filter 58. LNA 42 has an input for receiving the RF INPUT signal from an antenna or cable source (not shown), and an output. Up conversion mixer 44 has a first input connected to the output of LNA 42, a second input for receiving a signal labeled “LO1”, and an output. RF synthesizer 46 has first and second outputs for respectively providing first and second control signals. Oscillator 48 has an input connected to the first output of RF synthesizer 46, and an output for providing signal LO1. SAW filter 50 has an input connected to the output of mixer 44, and an output. PGA 52 has an input connected to the output of SAW filter 50, and an output. Mixer 54 has a first input connected to the output of PGA 52, a second input for receiving a signal labeled “LO2”, and an output. Oscillator 56 has an input connected to the second output of RF synthesizer 46, and an output connected to the second input of mixer 54. IF filter 58 has an input connected to the output of mixer 54, and an output for providing the IF OUTPUT signal with similar spectral characteristics as the output of tuner 20.
LNA 42 functions as a broadband amplifier and provides an amplified signal to mixer 44. Mixer 44 receives mixing signal LO1 from oscillator 48 at a frequency chosen to mix the selected channel to a frequency band centered around 1100 MHz, but mixes undesired channels as well. SAW filter 50 is an external filter that separates the desired channel, centered around 1100 MHz, from the unwanted channels. PGA 52 amplifies the output of SAW filter 50 to provide an output signal having uniform signal strength. Mixer 54 mixes the output of PGA 52 down to the desired IF frequency of 44 MHz using mixing signal LO2 at 1056 MHz provided by oscillator 56. IF filter 58 has a center frequency of 44 MHz and a passband of 6 MHz and attenuates unwanted channel information outside of this range.
While the up/down architecture of tuner 40 does not require manual calibration during manufacturing and is stable over temperature, it has many deficiencies that make its performance inferior to the discrete tuner illustrated in FIG. 1. Tuner 40 uses two high frequency oscillators. Because they are high frequency it is possible to implement them in silicon using inductor-capacitor (LC) oscillators. However LC-based oscillators have many drawbacks that reduce their desirability. First, they are susceptible to electric and magnetic interference which can create spurs (or tones) and noise and lower overall performance. Second, two oscillators which are close in frequency such as oscillators 48 and 56 used in tuner 40 tend to lock to one another. To avoid locking, there needs to be a lot of isolation between the two oscillators, which is difficult to achieve. Third, the first oscillator's range is nearly 100% of its frequency which means that the LC product must vary by about a 4:1 ratio to successfully tune over this range (since frequency is proportional to the square root of one over the LC product). However this range of values is difficult to achieve in silicon. Such an oscillator would usually be implemented as many selectable LC oscillators but this approach requires a lot of integrated circuit area. Fourth, having multiple LC oscillators adds phase noise which can degrade performance for digital television applications.
Another disadvantage relates to external SAW filter 50. SAW filter 50 is required because undesired channels need to be attenuated by a large amount and only SAW filters have the desired transfer characteristic at such high frequencies. However SAW filters are expensive. They need to be driven with a matched impedance, which increases power dissipation substantially. SAW filters are lossy. Also while SAW filters have good attenuation they have poor frequency selectivity and pass more than just the desired channel.
Another disadvantage relates to the mixing process in different signal environments. Cable television tuning requirements are very different from terrestrial television tuning requirements because of the difference in energy levels between a desired channel and undesired channels at adjacent frequencies. A cable head-end drives all channels with similar power levels and therefore a cable television tuner receives the desired and undesired channels at similar power levels. A terrestrial television receiver could be much closer to undesired channels' transmitters than to the desired channel's transmitter, leading to the undesired channels having much more signal energy than the desired channel. The tracking filter of tuner 20 helps filter the undesired channels. However since there is no tracking filter in tuner 40 and since SAW filter 50 passes more than the desired channel, mixers 44 and 54 see the large energy difference between the desired and the undesired channel.
This energy difference is very problematic since any spur or noise in the oscillator or non-linearity in the mixing process can mix the large undesired channel or channels into the desired channel and destroy the reception of the desired channel. The result is that tuner 40 has sufficient performance for some cable television applications, in which signal strength of all channels is nearly uniform, but poor performance as a terrestrial television tuner. Thus tuner 40 has failed to displace discrete tuner 20.
Thus it would be desirable to have a tuner architecture that is suitable for integration but which has performance comparable to or better than that of a discrete tuner.
Moreover certain transmission systems, such as broadcast television, frequently use direct conversion tuners with a high IF. A high IF has a frequency close to the desired frequency spectrum. North American broadcast television uses an IF of 44 MHz while the lowest channel is at 54 MHz, and thus they can be classified as high IF systems.
Traditionally broadcast television tuners do not use both high-side mixing and low-side mixing because they include tracking filters for image rejection. To keep the cost and complexity down of the tracking filters, only one type (high-side or low-side) of mixing is used. For various unrelated reasons, broadcast television tuners have standardized on high-side mixing. However, if image rejection is implemented without tracking filters, then both high-side and low-side mixing are possible without significantly increasing the cost or complexity of the tuner. High side mixers use an LO signal at the frequency of the desired channel plus the IF frequency. Using high side mixing reduces the oscillator frequency range compared to low side mixing, so that the LO can be practically implemented easily using conventional LC tank oscillators. In the case of North American broadcast television, the high side mixing signal used for terrestrial television varies from 98 MHz (44 MHz+54 MHz) to 850 MHz (806 MHz+44 MHz), and the tuning range from the minimum frequency to the maximum frequency is 850 MHz/98 MHz=8.7. If low side mixing were used, the LO signal would have to vary from 10 MHz (54 MHZ−44 MHz) to 762 MHz (806 MHz−44 MHz) and the tuning range from the minimum frequency to the maximum frequency would be 762 MHz/10 MHz=76.2. This tuning range is too large to be practically implemented using conventional LC tank oscillators, whose frequency of oscillation is determined by
      1          2      ⁢      π      ⁢              LC              .Thus a tuning range of 76 would require a variation in L or C of (76)2=5776.
However terrestrial television is characterized by a sparsely populated spectrum (e.g., less than 30 channels) in a wide bandwidth (54 MHz to 806 MHz in the U.S.). Due to the variable distance of each television receiver from the various transmitters, interfering channels vary in strength for each receiver. Also because different locations have different frequency allocations, interfering channels are at different frequencies for each television receiver. Thus some channels referred to as “blockers” create image frequencies that get mixed to the selected IF and are particularly troublesome because their signal energy can be much higher than that of the desired channel. For these blockers it would be desirable to use low side mixing, but low side mixing for high IF systems cannot be practically achieved using known LC tank oscillators and architectures having tuners with tracking filters.